Timing of global crustal metamorphism on Vesta as revealed by high-precision U–Pb dating and trace element chemistry of eucrite zircon

Abstract

Non-cumulate eucrites represent basaltic crust that experienced a complex thermal history involving multistage metamorphism and metasomatism, probably on asteroid Vesta. To better constrain the thermal history of these rocks and their parent body, we have integrated high-precision U-Pb age and trace element data for zircon grains with sizes up to 80 μm in the eucrite Agoult. All analyzed zircon grains yielded concordant U-Pb dates that correspond to the precise 207 Pb/ 206 Pb age of 4554.5 ± 2.0 Ma.The Ti contents in these zircon grains indicate their crystallization at subsolidus temperatures of ca. 900 • C, which are similar to the inferred conditions of pyroxene exsolution in most basaltic eucrites that occurred during protracted thermal metamorphism. The zircon crystallization temperatures, together with the presence of baddeleyite needles and variable Zr concentration in Agoult ilmenite grains, indicate metamorphic origin of the Agoult zircon through Zr release from ilmenite followed by reaction with silica. We therefore consider the zircon 207 Pb/ 206 Pb age as the timing of the widespread thermal metamorphism in Vesta's crust. The metamorphic age is coincident with the oldest Mn-Cr date for cumulate eucrites, supporting the view that the thermal metamorphism is a result of burial of basaltic crust and subsequent heating from the hot interior rather than collision of asteroids. The zircon rare earth element patterns with restricted Ce positive anomalies suggest that the metamorphism occurred at an oxygen fugacity below the iron-wüstite buffer, implying the absence of oxidizing agents such as aqueous fluid within the crust at that time.

Meteorite zircon constraints on the bulk Lu−Hf isotope composition and early differentiation of the Earth

“…Eight zircon grains were extracted from five thin sections of the eucrite Agoult. All eight zircon grains displayed no zoning structure with back-scattered electron and cathodoluminescence imaging (28). The grains were cleaned with 2 N HNO 3 and decomposed by concentrated HF + HNO 3 at 220°C.…”

“…Agoult is a highly metamorphosed basaltic eucrite, composed primarily of pyroxene and plagioclase with minor amounts of ilmenite, tridymite, troilite, spinel, Ca-phosphate, zircon, and baddeleyite (28,29). Zircon (ZrSiO 4 ) is typically associated with ilmenite and tridymite (SiO 2 ) (Fig.…”

In situ Pb‐Pb dating of silica‐rich Northwest Africa (NWA) 6594 basaltic eucrite and its constraint on thermal history of the Vestan crust

“…) or even earlier (Iizuka et al. ). Thermal metamorphism caused by burial of successive lava may overlap the peak of basaltic magmatism (4.55–4.56 Ga), because high eruption rate of magma is essential to result in a high enough peak metamorphic temperature that is usually observed in eucrites (Yamaguchi et al.…”

“…; Iizuka et al. ; Liao and Hsu ). However, thermal metamorphic temperatures of eucrites range between 700 and 1000 °C (Yamaguchi et al.…”

“…; Iizuka et al. ), such as burial and heating of a succession of lava flows as the crust grew by extrusive magmatism. The timing of thermal metamorphism is critical to explore the mechanism of metamorphism (i.e., burial by successive lava flows or reheating after magmatism) because their temporal implications are divergent.…”